Can you help me with my posture?

[Sascha]

Well I have like a bad posture and i dont like it (the top of my back is a tiny bit hunched but not as much as it used to be) and i have changed it a little bit but it is still hunched. The problem is that when i push my shoulders back to try and get my body into the right position a bone near the bottom of my ribs clicks. Its not only that my posture makes me feel like a weirdo (like everyone thinks im a hunchback) but its also when i sit down i get really bad cramps in my back and when i stand up for a long time (or hold something or run) i get bad cramps in my left shoulder. Its mainly just my should which gets bad cramps but my back hardly ever plays up but nearly every day at school i get shoulder cramps but not so much at home because i am more relaxed. Can you please help me to get a better posture and why are these things happening ? is it bad to change my posture or is it better for me? please help i hate it :/